Technical Field
This disclosure relates to an image forming apparatus.
Description of the Related Art
In general, electrophotographic image forming apparatus often cause a phenomenon such that due to deterioration of members of the developing device thereof and toner used for the developing device, mobility of the toner, which is imparted to the toner by the electric field formed between the toner supplying roller and the developing roller, deteriorates, thereby causing a problem (hereinafter referred to as a low density image problem or a faded image problem) such that the toner does not spread to the entire latent image area on the latent image bearer (such as photoconductor), resulting in formation of a low density toner image or a toner image having a low density area (i.e., faded image area) in the sub-scanning direction. In addition, when the environment surrounding the image forming apparatus (such as ambient temperature and humidity) changes or users change the image forming conditions of the image forming apparatus, the low density image problem tends to be caused.
In attempting to prevent occurrence of the problem, techniques such that by using a toner supplying roller characterized in property and configuration thereof or by properly controlling the bias (set value of bias) applied to a toner supplying roller, the power of the toner supplying roller to supply the toner to the developing roller is increased have been used. For example, there is a proposal in which by using the Paschen curve, the upper limit of the toner supply bias voltage is set to a voltage, at which discharge is not caused, to prevent formation of an image having uneven image density, which is formed due to abnormal electric discharge and leakage current. In addition, there is a proposal in which the ratio (d/∈) (i.e., equivalent dielectric thickness of the photoconductor) of the thickness of the photoconductor (d) to the specific dielectric constant (∈) of the photoconductor is set to 12 μm or less to prevent formation of background fouling.
There is another proposal concerning image forming method in which the supply bias is changed to produce high quality images even when the environmental conditions change. In this image forming method, an image pattern is formed at two or more positions of an electrostatic latent image bearer while changing the development bias, and the image densities of the image patterns are measured with an image density detector to determine the slope of the image density against the development bias, i.e., dependency of the image density on the development bias. The supply bias is changed based on the thus determined dependency to produce high quality images even when the environmental conditions change.